1. Field of the Invention
This invention relates generally to the field of listening devices. More specifically, the invention relates to novel personal listening devices with increased discernability and reduced listener fatigue.
2. Background of the Invention
The human ear is sensitive to sound pressure levels over 12 orders of magnitude. This broad range of sensitivity, which is measurable as discernability, is easily overwhelmed and restricted by the artificial sound and pressure concentrations extant in devices such as hearing aids, ear buds, in-the-ear monitors and headphones. This is different than mere sensitivity or susceptibility to overall volume levels. Discernability depends upon the ear's inherent ability to discern differences in sound pressure levels at different audio frequencies, relative to one another.
Conventional in-ear audio technologies occlude the ear canal to a greater or lesser degree with an ear mold, plug or other means of a device which contains a transducer and joins it to the canal, thereby creating a closed volume out of the ear canal itself. The ear is naturally suited to act as an impedance matching horn or Helmholtz resonator, not as a closed sound-vibration chamber. Occluding the ear canal with an audio transducer lowers the ear's discernability. Audio transducers comprise electromechanical mechanisms which involve greater mass and inertia than the delicate components of the inner ear. Directly coupling these to the tympanic membrane by creating a closed sound-vibration resonance chamber out of the ear canal markedly degrades the discernability of the ear by forcing it to emulate the transducer amplitude excursions as opposed to natural sound field excitations of the open ear.
Audio resonances, for example those occurring in environments such as rooms or the outdoors, are discernable to the unoccluded human ear. Blind persons have been known to effectively judge their proximity to environmental obstructions through acoustic differentiation based on changes in environmental sound sources external to the ear, which are perceived with the natural resonance of the open non-occluded ear. Closing the ear canal changes its natural open resonance condition (which is compensated for by the auditory system) to an unnatural hearing condition.
Even at very high sounds pressure levels above the threshold of pain in human hearing, the vibrational excursions of the tympanic membrane are not visible without the use of extreme magnification. In contrast, diaphragm excursions of conventional magnetic moving coil and moving armature devices are large and easily observed by the naked eye. Coupling such devices directly to the tympanic membrane by creating a closed sound-vibration chamber within the ear canal forces the tympanic membrane to emulate these same gross excursions and also to respond to average pressure changes in addition to sound pressures. This changes the natural vibrational modes and frequency response of the tympanic membrane and thereby inhibits its ability to differentiate sounds.
Personal listening devices have become extremely wide spread in recent years while physicians, audiologists and news agencies have continued to warn against hearing damage and old age deafness resulting from their use. These admonitions generally fail to delineate the specific mechanical factors causing such hearing loss and rather infer that listeners in general choose to listen to such devices at inordinate volume levels, or that these devices do unspecified damage despite reasonable use. Potential damage from choosing to listen at excessive volume levels is not limited to the use of in-ear or on-ear devices. Rather, the actual cause for concern is attributable to the fact that personal listening devices occlude the ear canal, thereby damping the tympanic membrane and reducing its sensitivity to audio vibrations, and further create a closed-canal pressure coupling of the audio transducer to the tympanic membrane which forces it to undergo unnaturally large excursions. Such abnormal excursions interrupt the normal tympanic modes of vibration, thereby rendering the ear even less sensitive and able to perceive sound naturally. The harmonic and other significant audio nuances of natural hearing are thereby lost and replaced by artificial membrane excitations whose audio resolution is insufficient to orient blind persons normally able to discern and navigate their environments by “seeing” with their unimpaired natural resort to louder volume levels in a futile effort to hear adequately. This is especially observable in cell phone and hearing aid users. In general use, prolonged exposure to these conditions may lead to permanent reductions in sensitivity and sound perception.
By simply forcing air through the Eustachian tubes into the middle ear volume repeatedly one can cause various over-excursions of the tympanic membrane. Hearing under these conditions is severely hampered. Just because the listener can still hear during the lesser tympanic over-excursions caused by conventional devices does not mean that he is hearing optimally. Due to the factors described above, audio fatigue from personal listening devices often occurs much sooner than it does with ambient sounds or even those produced by conventional loudspeakers in a concert or in a movie theater, given the same average volume levels.
In addition, the human auditory system incorporates mechanisms to reduce the acoustic input when levels become potentially damaging. The middle ear muscle reflex tightens the stapedius and tensor tympani muscles when loud sounds excite the hearing system. This reduces the amplitude of the vibrations conducted by the bones of the middle ear to the cochlea. The cochlea itself exhibits a threshold shift that reduces its neuronal output when stimulated by sustained loud sounds, at least in part due to the depletion of the available chemical energy. These mechanisms operate through the normal hearing pathway. Lowering the sound pressure in the ear canal reduces the chance of exciting these protection mechanisms that degrade the perception of sound.
Bone conduction provides another acoustic pathway to the hearing system, whereby sounds that vibrate the skull are able to excite the cochlea without a contribution from the tympanic membrane. It appears that increasing the mean or static pressure in the ear canal may modulate the effect of bone conduction and thereby alter the perceived sound. Conventional closed-canal devices modulate the static pressure in the ear canal and may contribute to this effect.
Although poor sound quality, audio fatigue and ear canal irritations are commonly associated with conventional in-ear devices, personal listening device audio transducers have been traditionally evaluated according to their performance relative to the acoustical impedance of air, measured in acoustic ohms according to Ohms Law. The primary problem is that once these audio transducers are partially or wholly sealed into the ear canal, the acoustic impedance of air is no longer applicable, the definitive factor now being the compressibility of air in a fixed volume. This confined air mass effectively transmits the energy of high amplitude transducer excursions to the ear drum. Hence the tympanic over-excursions, vibrational mode aberrations and occlusions described above are evidenced in all conventional prior art personal listening devices and hearing aids to greater or lesser degree.
Hearing aid manufacturers have resorted to porting their ear molds in an effort to overcome occlusion effects and the often overwhelming bass frequencies which occur when their devices form an acoustic seal of the ear canal. Personal listening devices such as ear buds utilize various methods of silicone, hollow polymer plugs, or foam which seal inconsistently, causing impaired audio performance as well as tissue pain from being repeatedly forced into uncomfortable positions by the user in an attempt to hear better. Custom molded devices such as in-the-ear stage monitors all create a closed chamber within the ear canal itself and suffer from the resulting audio degradations described above.
The aforementioned hearing aid porting only alleviates a small portion of the sound degradation attendant upon creating an artificial closed resonance chamber out of the ear canal. Hearing aids must maintain an adequate acoustic sealing of the ear canal in order to maintain isolation and prevent painful feedback conditions in which the device squeals or shrieks loudly as a consequence of the microphone repeatedly amplifying sounds which are meant to be contained in the acoustically sealed canal. Hence, the device remains mainly sealed and the ear canal is forced into becoming a closed resonance chamber. Extant devices, be they hearing aids, ear buds, or in-the-ear monitors, have no provision for containing their primary effective sound-vibration coupling chambers away from the tympanic membrane, and to this degree they limit and degrade the operation of the listener's ear regardless of the audio quality of the device. In addition to inhibiting the listener's own inherent discernability of sound, the abnormally large tympanic membrane excursions they cause are potentially physically damaging to the listener's hearing over time.
Additionally, isolation of the listener from the outside environment constitutes an annoying and often dangerous condition attendant upon the occlusion of the ear canal by conventional audio devices. When not posing a dangerous condition, conventional listing devices, limit the natural interaction between the listener and those about them. Those listening to music are normally cut off from external conversation, and often commonly complain of not being able to understand others.
Although breakthrough audio technologies often occur, they are limited by being applied in accordance with conventional in-ear speaker technology embodiments and do not compensate for the tympanic vibrational aberrations described above. Problems with user discomfort, occlusion, isolation, inadequate audio discernability and environmental orientation remain.
Consequently, there is a need for a personal listening device which reduces fatigue and possible damage to hearing associated with artificial pressure in the ear canal, and allows for the mixing of music or voice communications with outside sound to provide the listener with adequate environmental awareness, while improving discernability and the fidelity of the audio signal.